The linear motion rolling guide unit according to this invention has: a track rail(1); a casing(4) slidable relative to the track rail(1) and having a pair of raceway surfaces(11, 12); end caps(7) mounted to the casing(4); V-shaped grooves(15) formed between the raceway surfaces (11, 12) of the casing(4); a retainer(9) having an engagement projecting portion(16) engaged in the V-shaped groove(15); and a number of rollers(3) circulating in contact with the facing raceway surfaces 5 and 11, 6 and 12. The overall length of the retainer(9) is set larger than that of the casing(4), and the ends(19) of the retainer(9) are engagedin the engagement grooves(21) in the end caps(7) to position the end caps(7) with respect to the casing(4). Therefore, the positioning of the end caps(7) on the casing(4) can be made accurately without being affected byheat-treatment-induced deformation and machining errors.